1. Field of the Invention
This invention is concerned with apparatus for applying moisture to an object in predetermined incremental amounts. More particularly, this invention is directed to apparatus for applying incremental quantities of fluid to the printable surface of a recording medium in an electrolytic printer.
2. Description of the Prior Art
The concept of electrically generated printing has sparked interest since the 1840's. Most attempts at utilizing an electrically initiated printing reaction required relatively high voltages, in the order of 100 to 250 volts, saturated or partially wetted paper and/or consumable electrodes. It was also necessary to employ a recording medium which would be suitable for the particular printing system being used. Almost all of the prior art systems relied on either relatively high amplitude voltage pulses to achieve "dry" or "wet" printing. As might be expected, there were also hybrid systems and recording mediums therefor that attempted to reconcile and/or compensate for the disadvantages of both the dry and wet approaches. However, as is the case with most compromise situations, these hybrid systems were either too expensive to commercially implement or unsatisfactory in performance.
Various efforts were made to improve different aspects of the prior art electrically induced printing systems. Such efforts led to the use of electrolytic based printers wherein relatively low voltages, on the order of no more than 25 volts, are employed to effect printing. One printing system that functions at low energy printing levels, of a magnitude that is compatible with today's densely populated integrated circuit chips, is described in the commonly assigned U.S. patent application Ser. No. 237,560 filed on Feb. 24, 1981 in the name of Bernier et al. In this arrangement, a leuco dye resident in or applied to the surface of the recording medium used therein is rendered visible by the application of a low energy pulse thereto providing that the surface layer thickness of the recording medium, the contact surface area of the print electrodes and the spacing therebetween are all set to predetermined values.
It has been found that the use of electrical pulses to effect printing is substantially aided by moistening the surface layer of the recording medium, particularly in printers using pulses at low energy levels. U.S. Pat. No. 2,602,016 to Goldsmith sought to obtain an enhanced conditioning effect on the recording medium by applying steam thereto. In U.S. Pat. No. 3,890,622 to Alden, a premoistened recording medium is held in a sealed dispensing cassette until needed, thereby conserving moisture. Another strategy is evident in U.S. Pat. No. 3,974,041 to Haruta et al where zeolitic or moisture containing compounds are included within the surface layer of a recording medium, the moisture being liberated by the application of the print signal to the recording medium surface. Another, and more typical approach, is illustrated by the apparatus described in commonly assigned U.S. Pat. No. 4,335,967 to Pawletko where liquid is applied to the treated surface of a recording medium by a direct contact roller implemented applicator.
While operation of all electrically based printers, especially those using low energy pulses, would benefit from using one type or another of the prior art moistening approaches to improve recording medium conductivity and thereby lower energy requirements, these approaches would not be satisfactory for use in the Bernier et al type arrangement explained above for several reasons. The use of steam or other vapor would require additional energy and be incompatible with current printing systems. Further, unless careful handled, heated vapors would tend to pucker or mar the appearance of the final print copies. The use of a cassette or similar device to retain a quantity of premoistened recording medium suffers from its need to use a special and more expensive container that would also limit thruput speed. In addition, the degree to which the wetness of the premoistened recording medium held in the cassette was compatible with current operating conditions might require operator intervention in any event. The use of zeolytic compounds in the surface layer of the recording medium also suffers from most of the above-noted disadvantages. Such treatment of the recording medium would make it more expensive than the treated media now utilized, while still not guaranteeing that the moisture it produced would be commensurate with a given set of operating conditions. The direct roller application approach suffers from a potential contamination defect where any liquid returned to its reservoir may include some of the materials used to treat the surface layer of the recording medium. Lastly, all of the forgoing prior art approaches, in varying degrees over time under differing operating conditions, are unable to repeatedly apply accurate amounts of moisture at the exact locations where such moisture would be required.